C91 may refer to: Ruy Lopez chess openings ECO code Lymphoid leukemia ICD-10 code Brun C91, a 1991 sports prototype racing car built for Group C Dowagiac Municipal Airport in Dowagiac, Michigan FAA LID Paid Vacations (Seafarers) Convention (Revised), 1949 (shelved) code C91FM, a former branding name for Christchurch New Zealand radio C93FM Caldwell 91 (NGC 3532), an open cluster in the constellation Carina